Acid pickle tank covers

ABSTRACT

A cover assembly for a pickling line which includes a plurality of separate molded plastic cover sections; each section including a longitudinally extending top deck which is generally rectangular in shape. Adjacent the edges of the deck and substantially coextensive therewith, an an integral part of each cover section, are longitudinally extending plenum chambers of molded plastic, further including inwardly facing intake openings into said plenum chambers longitudinally along the length thereof for exhausting vapors released from the pickling line.

United States Patent Marnhall 1 51 Mar. 14, 1972 [54] MC) PICKLE TANK COVERS 2,325,158 7/1943 Wood ..l34/64 X 2,350,735 6/1944 Egge ..98/115 [72] Marshal" Pmsburgh' 2,467,505 9/1949 Sidell ..98/115 x [73] Assignee: The Ceilcote Company, Berea, Ohio Primary ExaminerWilliam F. ODea [22] Filed 1969 Assistant Examiner-P. D. Ferguson [21] Appl- 865,944 AttameyMeyer,Ti1berry and Body 52 us. Cl. ..98/115 n, 134/64, 266/3 R, [57] ABSTRACT 266/ 1 5 A cover assembly for a pickling line which includes a plurality [51] Int. Cl ..F23] 11/00 f Separate molded plastic cover Sections; each Section includ- [58] Fleld of Search ..98/42,43, 115; 134/64; ing a longitudinauy extending mp deck which i generany 266/3 3 15 rectangular in shape. Adjacent the edges of the deck and sub- 56 R stantially coextensive therewith, an an integral part of each 1 defences cued cover section, are longitudinally extending plenum chambers UNITED STATES PATENTS of molded plastic, further including inwardly facing intake openlngs into sald plenum chambers longltudlnally along the 1,553,569 9/1925 Stlck1e ..98/1l5 length th f f exhausting vapors released f k- 1,896,951 2/1933 Hahn ...93/1l5 1ing]ine 2,121,139 6/1938 Clauss et al.. ...l34/64 X 2,216,544 10/1940 True et a1. 134/64 4 Claims, 9 Drawing Figures Mmmmd March 114, K?

5 Sheets-Sheet 1 INVENTOR. ROBERT M. MARSHALL ATTORNEYS Mmmd March Mi, l72

3 Sheets-Sheet 2 INVENTOR. ROBERT M. MARSHALL ATDRNEYS ACID lPItClltLlE TANK CUi/lElltS The present inventionpertains to a cover assembly, and more particularly to a cover assembly which has as an integral part thereof means for collecting and exhausting toxic and/or corrosive fumes released from a pickling line.

The invention is particularly applicable to a continuous strip acid pickling line, and will be described with particular reference thereto, although it will be appreciated that the invention has broader applications, such as the collection of toxic or corrosive fumes released in other types of process lines or systems.

A continuous strip pickling line uses a plurality of acid tanks in end-to-end relationship with means to pass steel strip material in a longitudinal direction through the tanks for acid pickling.

The lines are long, for instance, 360 feet in length, comprising perhaps four tanks each 90 feet in length. Heretofore, it has been conventional practice to cover the successive tanks with metal (steel) neoprene-lined covers. Usually the tanks are provided with a plurality of draft boxes which extend the full length of the tanks and are seated on top of the sides of the tanks, the covers resting upon or being seated on top of the draft boxes. As with the covers, the draft boxes also are made of steel and are lined with a material such as neoprene. Openings are provided in the draft boxes facing inwardly, in the direction of the acid pickling baths, and the boxes are connected at suitable points to a plurality of exhaust ducts so as to exhaust acid fumes from beneath the covers, and from above the acid baths.

The acid which has been conventionally used for pickling is sulfuric acid. There is only a slight or no fuming of sulfuric acid, and the rubber lined metal covers and draft boxes, although subject to corrosion, lasted relatively well with minimum replacement costs and downtime. The toxicity of whatever sulfuric acid is released to the atmosphere from the acid baths is so slight that it is not unknown for an operator to remove and even dispense with the line covers during operation of the line.

In recent years, sulfuric acid has become more expensive, with a shortage of sulfur, so that more and more, the steel industry has gone to hydrochloric acid for the pickling operation. One advantage of hydrochloric acid is that with the use of substantially higher temperatures, the speed of the pickling line can be substantially increased. The temperatures employed are as high as 190 F. to 200 F. at the inlet end of the pickling line dropping to about 160 IF. adjacent to the outlet end.

These higher temperatures result in substantial fuming of the hydrochloric acid, considerably more than that experienced with sulfuric acid.

In addition hydrochloric acid is substantially more corrosive than sulfuric acid. For both reasons, the greater quantity of acid fumes and higher degree of corrosiveness, it was quickly found that the conventionally employed neoprene lined steel covers and draft boxes were unable to hold up, or endure, the neoprene debonding from the metal, and of course, the metal corroding.

It has been proposed to coat the steel covers and draft boxes with natural rubber, a more expensive operation, but natural rubber also deteriorates when subjected to the hydrochloric acid fumes.

If a cover is corroded, it simply is lifted from the line and replaced with another cover; but to replace or reline a draft box, the line must be shut down. For a long unit and because of the complexity of replacing or relining a draft box, a line often will be shut down a substantial period of time to perform these operations.

It has been proposed to use various plastics for construction of the pickling line covers, and polypropylene and fiber glass resins in particular have received significant acceptance in the industry. Both plastics have a long life in the presence of hydrochloric acid fumes.

However, polypropylene cannot be used for construction of the draft boxes. The reason for this is the acid tanks conventionally are constructed of steel lined internally with several courses of acid brick, and acid brick has an expansion and contraction perfoot with changes in temperature close to that of steel, but substantially less than the expansion and contraction per foot of polypropylene with changes in temperature.

0f necessity the draft boxes must be securely fastened or anchored to the sides of the pickling line acid tanks, to seal the space between the draft boxes and top surfaces of the acid tank sides. In the absence of such sealing, an excessively large draft or airflow is required in the fume collection system to prevent the leakage of toxic acid fumes to the surrounding atmosphere through the leakage points which would exist between the draft boxes and tank sides. Leakage points already exist in a pickling line at points where penetrations are made in the line cover or draft boxes for such accessories as acid feed and steam jet conduits, and it obviously is important to minimize as much as possible the overall area through which leaks can occur; and thus the draft or airflow required. This is particularly important in connection with hydrochloric acid because of the fuming and toxicity of this acid.

The problem is more acute with differential contraction than expansion. When the draft boxes have been constructed of plastic materials and anchored to the acid tank sides, there seems to be enough relative give in the component parts to accommodate the difference in expansion. However, polypropylene has a relatively low tensile strength, and when used for draft box construction, the boxes have quickly been pulled apart during cooling of the acid baths and following use, because of the greater contraction of the draft boxes relative to the tank sides to which they areattached. The problem is aggravated by the higher temperatures employed with the use of hydrochloric acid, as well as by the substantial lengths of the pickling lines. Fiber glass reinforced resins have been successfully employed in approximateIyAO-foot sections for the construction of draft boxes. In this type of installation the draft boxes are not anchored to the tank sides.

Accordingly, it is an object of the present invention to overcome the above problems, and in particular to provide a novel safe fume collection system which is resistant to high temperature corrosive hydrochloric acid fumes.

It is further an object of the invention to provide a cover assembly and fume collection system for a pickling line in which the overall area of leakage is minimized, thereby reducing the size of the acid fume exhaust and disposal system required.

It also is an object of the invention to provide an acid resistant fume collection system which is not subject to destruction from differential expansion and contraction with the acid tanks.

In accordance with the invention there is provided a cover assembly for an acid pickling line which includes a plurality of separate molded plastic cover sections; each section comprising a top deck which is generally rectangular in shape; and edge portion substantially coextensive with the top deck, and opposed longitudinally extending plenum chambers adjacent to and substantially coextensive with said edge portion, said plenum chambers being an integral part of each section; further including inwardly facing intake openings in said plenum chambers longitudinally along the length thereof to exhaust vapors released from said pickling.

Preferably, each of the cover sections comprises at the ends thereof a tongue-and-groove type of construction for mating one section with another, the plenum chambers of successive sections being aligned and open at the ends thereof so that the chambers are substantially continuous along the length of the pickling line.

Also, in accordance with a preferred aspect of the invention, each cover section includes adjacent at least one end thereof laterally extending duct means in communication with the plenum chambers, said duct means being an integral part of said cover section and including means defining laterally disposed intake openings to exhaust vapors released from the pickling line in the area of connection or joining of successive cover sections.

Further in accordance with the invention, the plenum chambers are in communication with a plurality of collector ducts positioned at spaced intervals along both sides of the pickling line, the acid fumes in the plenum chambers being transmitted via the collector ducts to suitable disposal means.

Preferably the cover assembly comprises a plurality of short collector duct sections similar to the cover sections in shape and configuration, but of substantially shorter length. The collector duct sections are interspersed at spaced intervals between the cover sections, and each supports, as an integral part thereof, opposed laterally extending collector ducts which are in communication with the plenum chambers of the cover sections.

As a further preferred aspect, the intake openings in the plenum chambers are progressively increased in size the further they are spaced from the collector ducts to obtain a uniform removal of fumes lengthwise of the cover assembly. In areas where the cover assembly is penetrated by accessories for the pickle line, the intake openings are also enlarged.

It will become apparent that by replacing the conventional draft boxes with plenum chambers which are an integral part of the cover sections, and constructing the plenum chambers of the same acid resistant plastic materials used in the cover sections the problem of downtime for replacement and relining of draft boxes is eliminated. It also will become apparent that by using plenum chambers which are an integral part of the cover sections, the overall area for leakage is minimized permitting optimum minimum sizing of the plenum chambers, as well as the acid disposal system with which the plenum chambers are connected. Further eliminated is the problem of destruction of plastic draft boxes because of differential expansion and contraction of component parts. The plenum chambers being an integral part of the cover sections simply expand with the latter, the cover sections seating on the upstanding walls of the acid tanks in sliding engagement therewith.

As an additional advantage, the fume collection system in accordance with the invention is part of a plurality of small self-contained sections which, if one is damaged, can be removed and replaced with ease and without significant downtime.

As a further advantage, the plenum chambers, being an integral part of the cover sections, constitute a means for cover reinforcement, as well as a means for fume removal.

The invention, and other advantages thereof, will become apparent upon consideration of the following specification, with reference to the accompanying drawings, in which FIG. 1 is a schematic section elevation view of a continuous strip pickling line in accordance with the present invention;

FIG. 2 is an enlarged section view taken through line 2-2 of FIG. 1;

FIG. 3 is an enlarged partial section view of a pickle tank cover assembly in accordance with the present invention;

FIG. 4 is a section view taken through line 4-4 of FIG. 3;

FIG. 5 is an enlarged detail section view showing details of the connection means between successive cover sections of a cover assembly in accordance with the invention;

FIG. 6 is a section view through line 6-6 of FIG. 3;

FIG. 7 is a top plan view of the cover assembly of FIG. 3 in accordance with the invention;

FIG. 8 is a perspective end view of one cover section of a pickle tank cover assembly in accordance with the invention; and

FIG. 9 illustrates an embodiment in accordance with the invention.

Referring now to the drawings wherein the showings are for the purpose of illustrating the preferred embodiment of the invention only and not for the purposes of limiting the same, the FIGURES show a continuous strip pickling line A which is covered with a pickle tank cover assembly B. Part of the pickling line is a fume exhaust system C including collector ducts D to exhaust fumes from the space beneath the cover assembly B and above the baths E of pickling acid.

Referring to the FIGURES in detail, particularly FIGS. 1 and 2, the continuous strip pickling line comprises a plurality of acid pickle tanks 10, in this instance two, each tank being constructed of steel lined with acid brick, perhaps three courses of brick. The tanks are provided with a floor 12, side walls 14 (FIG. 2') and end walls 16, and in each of the tanks is a bath of acid 18 at temperatures ranging from about 190 to 200 at the inlet end 20 dropping to about 160 at the outlet end 22. Although there are only two tanks shown in the drawing of FIG. 1, obviously a greater number of tanks can be used, for instance four tanks, each having a length of about feet. It is to be appreciated that the subject invention is suitable for use with a variety of pickling acids such as, hydrochloric, sulfuric, nitric and hydrofloric acid.

A strip of material to be pickled is fed from inlet rollers 24 over the leading lip 26 of the first tank, dipping through the first bath 28, over a pair of intermediate rollers 30 at the junction of the first and second tanks, dipping into the second bath 32, and then passing over a lip 34 at the outlet end of the second tank for passage through a rinse area 36.

The cover assembly B for the acid tanks extends longitudinally over the two baths 28 and 32, the assembly comprising a plurality of successive cover sections 38 which are joined together at points 40 in a manner to be described.

Also illustrated in FIG. 1 are conduits 42 for acid feed, or steam injection, and strip lifter means 44, a plurality of each of these items being disposed at intervals along the length of the pickling line as required. Openings in the cover assembly are provided to accommodate the items. The fume exhaust system C for the pickling line, in addition to the collector ducts D which lead from the space between the surface of the acid bath and cover assembly, includes a plurality of downcomers 46 at spaced intervals on each side of the pickling line connected to each collector duct, and an exhaust manifold 48 leading to a fume disposal means schematically shown as item 50. It is to be appreciated that the collector ducts D could be connected above the cover assembly B, in which case downcomers 46 would be required only on one side of the pickling line for connection to the exhaust manifold 48.

In FIG. 2, the concepts of the invention are illustrated in slightly more detail. Each cover section 38 of the cover assembly B is provided with a generally rectangular upwardly bowed roof or top deck 52 and downwardly and longitudinally extending side wall portions 54 which are integral and coextensive with the top deck. These side wall portions may be of any height or may be eliminated entirely. The lower edges of the side wall portions 54 terminate in a flange 58 which seats on the upwardly facing shoulders 56 of the side walls 14 of the acid pickle tanks 10. Integral with the top deck and forming an integral part of each cover section, are opposed longitudinally extending plenum chambers 60 which are adjacent to the side wall portions 54, and coextensive therewith but are integral with or part of the top deck 52. These chambers may be located above or below the deck.

More details of the cover assembly can be seen with reference to FIGS. 3, 4, 5, and 6. Each of the successive sections 38 has an inverted box or shell-like configuration; the top deck 52 being bowed upwardly relative to the downwardly extending side walls for the run-off of moisture which collects on the roof. The horizontally facing flanges 58 for the side wall portions of the cover assembly are provided with longitudinally extending strips 62 of sealing means long the lower surfaces thereof, of any composition which is slightly resilient, to seal the space between the cover assembly flanges 58 and the shoulders 56 of the tank walls. The sealing strips 62 also are of a material which is resistant to acid fumes. At spaced intervals along the top deck, the cover sections are provided with lifting lugs 64 for crane removal and replacement of the sections. It is to be appreciated however, that the cover sections can also be removed by means of air or hydraulic cylinders. To complete the inverted shell or boxlike configuration each cover section has opposed downwardly extending end walls or ribs 66 which extend between the cover side walls 54.

FIG. 5 illustrates the manner in which the successive sections are joined or brought together into sealing engagement. The sections can be described generally as having a tongueand-groove type construction, one section having exterior roof wall and side wall flanges 68, 70 which extend beyond the end wall 66 of the section, and which slide over and engage the exterior roof and side wall surfaces 72, 74l of the end of the other section. The exterior flanges 66, 76 of the one section are dimensioned so as to accommodate a sealing material 76 against which the exterior surfaces 72, 74, of the other section are positioned. Preferably each cover section 36 would have the plain surfaces 72, 74 at one end and the flange surfaces 68, 70 at the other end. When the successive sections are brought together, the end walls 66 thereof are contiguous or in abutting relationship.

One purpose of the end walls is that they constitute strengthening ribs for the ends of the cover sections. A Ion gitudinally extending rib 77 (FIG. 4) also is provided integral with the roof portion on the underside thereof and positioned between the opposed side wall portions 54.

The plenum chambers 60 are half-round in configuration and extend the full length of the cover sections along the opposite sides thereof. The abutting end walls 66 of the cover sections are provided with cutouts or openings 80, clearly shown in FIG. fl, aligned with each plenum chamber, so that when one section 3% is abutted against another section, the respective plenum chambers of the two sections are in alignment and in flow communication with each other. In this way, the plenum chambers together define continuous passages along both sides of the pickle line which extend substantially the entire length of the pickle line. Although the plenum chambers 66 are shown in the form of half-round cross sec tions square or rectangular cross sections would work equally as well.

Each of the plenum chambers 60 is provided with an intake slot or openinghll which runs substantially its entire length, the slots or openings 61 being downwardly directed and oriented slightly towards the side wall portions 54 of the cover sections, to pick up fumes which would normally escape between the side wall portions of the cover sections and the upper shoulders 56 of the acid tank walls. The slots or openings fill can be from one-sixteenth inches in width up, depending upon the width of the pickle tanks and the amount of acid fumes to be removed.

Instead of continuous slots or openings, the plenum chambers can be provided with a plurality of holes, an important feature being that they are downwardly oriented, to allow acid which may condense in the plenum chambers to drain back into the pickle tanks, eliminating a hazzard to personnel or operators of the pickling line.

In order to exhaust or remove acid fumes which would normally escape between the ends of the cover sections, there are provided smaller transverse flue ducts 82 which are integral with and formed along the inside surfaces of at least one of the end walls of each section. These smaller flue ducts are provided with %inch or 16-inch intake openings 64 at spaced intervals in a transverse direction, the openings being downwardly oriented to exhaust fumes in the area of connection between the cover sections. Obviously, the transverse ducts could be slotted similar to the plenum chambers, or provided with larger or smaller openings, in accordance with design criteria.

At spaced intervals along the cover assembly, the collector ducts D are located into which the plenum chambers 60 open. In the example shown in FIGS. 3-6, the collector ducts are integral with and part of short collector duct sections 86 of about 4 feet in length, each small section being provided with tongue and groove ends, similar to the ends for the regular cover sections, of longer length which contain the plenum chambers, for insertion between the regular cover sections. As an alternative, the collector ducts can be made an integral part of the regular cover sections.

The collector ducts D, as shown in FIGS, 3, 4i and 6, are L- shaped members disposed on opposite sides of each collector duct section each having an upstanding leg 66 which is affixed to the top deck 52 of the section, adjacent to the edges thereof, and horizontal and downwardly extending legs and 92, the latter being connected to the downcorners 46 of the fume collection system. Each collector duct section along the sides thereof is provided with short passageways or plenum chamber portions 94, similar to those of the longer cover sections, except that the top deck is cutout in areas for substantially the entire length of the section (notice FIG. 7) and the legs 83 are pyramid shaped (FIGS. 6 and 7) forming a sloped umbrella over the cutout areas; so that the legs have a substantial area in communication with the plenum chambers portions 94. The end walls 66 of the collector duct sections are also cutout, at areas in alignment with the plenum chambers thereof, as with the longer cover sections, so that the collector ducts on each side are in flow communication with all of the cover plenum chambers on that side of the cover assembly.

Reinforcement means in the form of a pair of angles 96 are provided bracing the collector ducts to the top decks of the collector duct sections.

0n assembly, the collector duct sections, of which the collector ducts are an integral part, are strategically positioned along the pickle line so that the downwardly extending legs of the collector ducts mate with the downcorners of the pickle line fume collection system. Preferably, the short or small sections of the cover assembly are secured to the acid tanks or fume collection system, and would seldom be moved.

In FIG. 3, there are shown cutout areas 98 for acid, water and steam jet conduits or pipes 42, FIG. 4, at spaced intervals in the side wall portions 5% of the cover sections. The cutout areas are U-shaped openings, sufficiently large to accommodate the acid, water and steam jet conduits, but are dimensioned as small as possible to minimize the clearance in the cutout areas for the escape of acid fumes. The cutout areas are spaced slightly below the plenum chambers, and the intake openings or slots fill in the plenum chambers in the proximity of the cutout areas are slightly enlarged to draw air inwardly through the clearance between the side walls of the cover sections and the conduits penetrating the side walls, to avoid dispersion of acid fumes outside of the: acid tanks.

FIG. 33, also shows a larger cutout area 100 for the strip lifters M. ln this instance, the cutout area extends upwardly to the elevation of the plenum chambers, and inwardly into the top deck or roof portion of the cover sections. This is also shown in FIG. 7, the penetration into the top deck or roof portion being substantial. To accommodate this penetration, the plenum chambers are looped or bypassed inwardly, as shown in FIG. 7, at points 102, so that the plenum chambers define substantially continuous flue ducts along the entire length of the pickle tank line.

As with the cutouts for the acid, water and steam jets, the clearance at areas lltltl is minimized, and in addition, the intake openings or slots 81 in the plenum chambers are enlarged in the area of the strip lifter cutouts to prevent leakage of fumes in the clearance spaces around the strip lifters.

An alternative in accordance with the present invention is illustrated in FIG. 9, wherein the plenum chambers 11M are disposed longitudinally adjacent the sides of the cover sections but integral with and on the upper surfaces of the top decks thereof. Openings in this embodiment are longitudinally extending slots W6 provided in the top deck between the plenum chambers and areas above the acid baths.

Not shown in FIG. 9 is an arrangement by which moisture collecting on the top deck is allowed to drain from the deck. Suitable enclosed channels can be provided penetrating the plenum chambers at spaced intervals.

Most large pickling lines will utilize about 16 collector ducts, so that each duct handles about 40 feet of plenum chamber, although it is obvious that this can vary depending upon design considerations.

ln practice, the cover sections and fume collection system will be fabricated from a thermoplastic or thermoset material, possibly reinforced with fiber glass. A suitable plastic is polyproplene, which is corrosion resistant to hydrochloric acid fumes.

in manufacture, the plenum chambers, collector ducts, and transverse end wall flue ducts would be fabricated or molded separately from the cover sections, and then simply welded to the cover sections by the use of known filler rod welding processes.

An advantage of the invention is that the cover sections are suitable for newly constructed pickling lines, or for replacement of existing covers and flue systems for existing pickling lines. In the latter instance, the existing structures are simply removed, and the cover sections are positioned in place with the flanges 58 resting upon the upper surfaces of the pickling tank sides. It is a simple matter to dimension the cover sections to locate the collector ducts at downcomers for the existing fume removal system, made simpler by employing small sections for the collector ducts, between larger cover sections for the rest of the pickling line. Most important, no provision need be made for differences in expansion between the cover assembly, fume collection system, and acid tanks.

Whether the cover assembly is for an existing line or new line, it is apparent that the problem of downtime for replacement or relining of draft boxes is avoided.

Because the plenum chambers, collector ducts, and other components are an integral part of the cover sections, the weight of the sections is sufficient to provide excellent sealing between the cover sections and the tank sides.

This and the elimination of other potential leakage points, by the provision of an integral assembly, increases the efficiency of acid fume removal, resulting in a reduction in the size or capacity of the acid fume handling system which is required.

In this respect, by enlarging the plenum chamber intake slots or openings 80 in the proximity of penetrations made in the cover assembly for accessories, to pull a special vacuum at these penetrations, the efficiency of acid fume removal is increased reducing air flow requirements for the exhaust system.

Particularly in new installations, the exhaust system can be reduced in size, reducing the capital costs involved in a pickling line installation.

As a further advantage, in accordance with the invention, since the cover assembly is in the form of self-contained small sections, it is a simple matter to remove a section which is damaged and replace it with a substitute section, avoiding substantial maintenance costs and downtime.

As an additional advantage, the plenum chambers being an integral part of the cover sections, act as reinforcement means for the cover sections.

Although the invention has been describe with reference to specific embodiments, variations within the scope of the following claims will be apparent to those skilled in the art.

What I claim is:

l. A tank cover assembly including tanks containing a liquid which emits noxious fumes, said tank being subject to thermal expansion and contraction and having opposed longitudinally extending upstanding tank side walls, comprising:

a plurality of separate plastic cover sections;

each section comprising a roof portion with opposed longitudinally extending side edge portions seated in slidable engagement upon the upper portion of said tank side walls whereby said tank side walls and said side edge portions are free to thermally expand and contract at respectively different rates;

each section further comprising first and second opposed end walls extending between said side edges;

two opposed plenum chambers extending parallel to and adjacent said side edge portions, the plenum chambers being an integral part of said sections;

inwardly facing intake openings in said plenum chambers disposed longitudinally along the length thereof in flow communication with fumes emitted from said liquids and laterally extending exhaust ducts ad acent at least one end wall of each cover section and integral therewith to exhaust fumes from said liquid in the vicinity of the joint between successive sections.

2. A tank cover assembly including tanks containing a liquid which emits noxious fumes, said tanks having opposed, longitudinally extending upstanding side walls, comprising a plurality of separate plastic cover sections, each section comprising a roof portion with opposed longitudinally extending side edge portions seated upon the upper portions of said tank side walls,

two opposed plenum chambers extending parallel to and adjacent said side edge portions, the plenum chambers being an integral portion of said sections,

inwardly facing intake openings in said plenum chambers disposed longitudinally along the length thereof in flow communication with fumes emitted from said liquid, and laterally extending ducts adjacent at least one end wall of each cover section and integral therewith to exhaust fumes from said liquid in the vicinity of the joint between successive'sections.

3. The cover assembly of claim 2 wherein said cover sections include opposed side wall portions extending downwardly from said opposed edge portions.

4. The cover assembly of claim 2 wherein said tank is an acid pickle tank for a continuous pickling line. 

1. A tank cover assembly including tanks containing a liquid which emits noxious fumes, said tank being subject to thermal expansion and contraction and having opposed longitudinally extending upstanding tank side walls, comprising: a plurality of separate plastic cover sections; each section comprising a roof portion with opposed longitudinally extending side edge portions seated in slidable engagement upon the upper portion of said tank side walls whereby said tank side walls and said side edge portions are free to thermally expand and contract at repsectively different rates; each section further comprising first and second opposed end walls extending between said side edges; two opposed plenum chambers extending parallel to and adjacent said side edge portions, the plenum chambers being an integral part of said sections; inwardly facing intake openings in said plenum chambers disposed longitudinally along the length thereof in flow communication with fumes emitted from said liquids and laterally extending exhaust ducts adjacent at least one end wall of each cover section and integral therewith to exhaust fumes from said liquid in the vicinity of the joint between successive sections.
 2. A tank cover assembly including tanks containing a liquid which emits noxious fumes, said tanks having opposed, longitudinally extending upstanding side walls, comprising a plurality of separate plastic cover sections, each section comprising a roof portion with opposed longitudinally extending side edge portions seated upon the upper portions of said tank side walls, two opposed plenum chambers extending parallel to and adjacent said side edge portions, the plenum chambers being an integral portion of said sections, inwardly facing intake openings in said plenum chambers disposed longitudinally along the length thereof in flow communication with fumes emitted from said liquid, and laterally extending ducts adjacent at least one end wall of each cover section and integral therewith to exhaust fumes from said liquid in the vicinity of the joint between successive sections.
 3. The cover assembly of claim 2 wherein said cover sections include opposed side wall portions extending downwardly from said opposed edge portions.
 4. The cover assembly of claim 2 wherein said tank is an acid pickle tank for a continuous pickling line. 